Factors affecting the exchange of genetic material between Nordic and US Holstein populations.

The possibility of profitable cooperation between dairy cattle populations depends on several factors. Among these factors is the similarity of breeding goals, for example, as measured by the correlations between selection indices. Correlations between selection indices less than unity can usually be explained by differences in economic values, trait definitions, national genetic evaluation procedures, and genotype x environment interactions. The objective of this study was to test whether uniform definitions of the female fertility traits would increase the exchange of genes across populations, and to quantify the effect on genetic gain. A second objective was to test whether a more similar relative weighting of the index traits across populations would increase the exchange of genes across populations, and to quantify the effect on genetic gain. This was done in a stochastic simulation study of the Nordic and US Holstein populations. Uniform definitions of the female fertility traits did not increase total genetic gain in the Nordic Holstein population. The standardization did not seem to affect selection across populations either. However, the results were sensitive to the assumptions made in the simulation study, especially the genetic correlations between traits. A more similar relative weighting of the index traits across populations did not change total genetic gain in the Nordic Holstein population. The possibility of exchanging genetic material with the US Holstein population led to significantly higher progress in the aggregate genotype in the Nordic Holstein population compared with a situation in which exchange was not possible. Hence, importation of US Holstein genetics for use in the Nordic Holstein population is recommended. In addition, results indicated that population size is of greater importance than differences in trait definitions and relative weighting of the index traits for the advantage of exchanging genetic material between the Nordic and the US Holstein populations. The possibility of exchanging genetic material with the Nordic Holstein population did not change progress in the aggregate genotype in the US Holstein population compared with a situation in which exchange was not possible, but it tended to result in lower genetic progress in protein yield and greater genetic progress or smaller genetic declines in the functional traits. Thus, importation of genetic material from Nordic Holsteins may slow down the deterioration of animal health and reproduction in US Holsteins.